Graphite crucible test method

ABSTRACT

The detection of undesirably large pores or fissures in graphite crucibles to be used in a high-temperature vacuum metallizing process which can cause premature cracking of the crucible, by immersing the crucible in water, drawing a vacuum over the water surface and observing the presence of any large diameter bubbles.

United States Patent inventor John T. Carleton Williamsville, N.Y.

Appl. No. 853,197

Filed Aug. 26, 1969 Patented July 13, 1971 Assignee E. I. du Pont de Nemours and Company Wilmington, Del.

GRAPHITE CRUCIBLE TEST METHOD 1 Claim, No Drawings US. Cl 73/37, 73/455 lnt. CL G0lm 3/06 Field of Search .4 73/37, 40, 38, 49.2, 45.5

Primary Examiner-Louis R. Prince Assistant Examiner-William A. Henry, II AnorneyFrancis A. Paintin ABSTRACT: The detection of undesirably large pores or fissures in graphite crucibles to be used in a high-temperature vacuum metallizing process which can cause premature cracking of the crucible, by immersing the crucible in water, drawing a vacuum over the water surface and observing the presence of any large diameter bubbles.

GRAPHITE CRUCIBLE TEST METHOD BACKGROU N D OF THE INVENTION This invention relates to crucibles ordinarily employed in vacuum induction heating processes such as vacuum metallizing, and more particularly, to methods of testing such crucibles.

The processes wherein thin layers of metals, such as aluminum, selenium, copper, or tin, are deposited on films or similar surfaces are well known in the art. It is common in such processes to use graphite crucibles to contain the molten metal. Since graphite is known to be somewhat porous, there is likely to be some penetration of these pores by the molten metal. If the pores are of substantial diameter, rather significant amounts of the metal can seep therein. When the crucible is heated (e.g., in the range of 1,000 to 1600 C.) and cooled, the metal in the large pores can undergo expansions and contractions at a rate different from the graphite and cause cracking of the crucible making replacement necessary. It is possible that some chemical reactions that may take place also contribute to this deterioration. It is not uncommon for some crucibles to last not more than an hour of effective operation before breakage necessitates replacement.

Although the graphite from which the crucibles are made is frequently tested by the manufacturer for porosity, it is common only to test a small test sample. Unfortunately, a single pore or fissure of substantial size anywhere throughout the length of the crucible can cause cracking and render the entire piece unusable. Applicant has discovered a method of testing crucibles to locate sizable pores or fissures if they exist anywhere on the crucible and thereby determine in advance of use whether or not a given crucible will survive a sufficient number of hours of operation to render its use economically satisfactory.

SUMMARY OF THE INVENTION Briefly stated, the invention comprises a method of testing graphite crucibles for the presence of undesirably large pores or fissures, which method comprises immersing the crucible in water, reducing the atmospheric pressure over the surface of the water to a point where bubbles are observed coming from the pores in the graphite surface and visually determining the existence of any pores which yield air bubbles of a diameter sufficient to indicate an undesirably large pore or fissure in the crucible.

DETAILED DESCRIPTION OF THE INVENTION As stated before, this method can be used in conjunction with a vacuum metallizing process of aluminum or the like, such as is generally described in Steeves, U.S. Pat. No. 3,227,431, Chadsey et al., U.S. Pats. No. 2,643,201 and No. 2,665,320, Clough, U.S. Pat. No. 2,665,223 and Bancroft, U.S. Pat. No. 2,584,660.

The invention will now be described in connection with a specific example thereof.

EXAMPLE A graphite crucible is obtained which was prepared from a semicylindrical solid piece of graphite 15.5 inches long and 5 inches in diameter and hollowed out to provide a trough with a wall thickness of about 0.75 inch. The internal corners in the crucible trough are rounded off. The crucible so prepared is placed in a glass jar about 10 inches in diameter and about 18 inches high filled with degassed water. The jar is provided with a conventional vacuumtight cover with conventional means of drawing a vacuum on the air space above the water in the jar. The crucible is immersed entirely in the water and the cover is placed on the jar and a vacuum is pulled reducing the pressure above the water to about 5 to 10 p.s.i. absolute One observes the extremely small streams of bubbles over the entire surface of the crucible indicating the natural porosity of the graphite and the air trapped therewithin. These bubb es are generally of less than about one-sixteenth inch in diameter under the conditions described in this example. The observance of one or more streams of bubbles whose diameter is greater than about one-eighth inch in diameter under the conditions of this example indicates the presence of one or more pores or fissures of sufficient size to admit a sufficient amount of metal during a typical aluminum metallization process to cause a failure in from about 1 to 4 hours of operation of ordinary use of the crucible in a vacuum aluminum metallizing process such as described in the prior art, e.g., the Steeves patent. If, however, no large bubble streams are observed and the streams are all made up of bubbles about one-sixteenth inch or less in diameter, the crucible is substantially free of undesirably large pores or fissures and should last 8 to 16 hours or even longer in typical vacuum metallizing processes wherein typical temperatures are of the order of l,400 C.

The bubble sizes indicated in this example are only approximations made under the conditions set forth. Naturally, if different vacuum conditions or water depths are involved, the bubble diameters may vary, but it is well within the skill of the art to determine suitable observation criteria to carry out the invention under many different conditions.

As described in the prior art, e.g., Baer, U.S. Pat. No. 3,084,060, it may be desirable to coat the crucible before use in the metallizing process. In that event, the crucible should be tested before the coating is applied.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims, and all changes which come within the meaning and range of equivalence are intended to be embraced therein.

Iclaim:

1. In a vacuum metallizing process wherein a molten metal is vacuum metallized from a graphite crucible at temperatures in the range of 1,000 to 1,600 C., the improvement of pretesting the graphite crucible by immersing the crucible in water, reducing the atmospheric pressure over the surface of the water to about 5 to 10 p.s.i. absolute and visually determining the existence of any pores or fissures in the crucible which yield air bubbles of a diameter of the order of oneeighth inch or larger. 

